A battery unit includes a plurality of batteries which are electrically connected (Patent Publication 1).
Each of the batteries constituting the battery unit includes, for example, a battery case having an opening, a sealing plate for sealing the opening, and an electrode group placed in the battery case. A positive electrode of the electrode group is connected to a positive electrode terminal of the battery (e.g., the sealing plate), and a negative electrode of the electrode group is connected to a negative electrode terminal of the battery (e.g., the battery case).